The regulation of murine B lymphocytes by signals delivered through their surface immunoglobulin receptors will be studied. Emphasis will be placed upon the monoclonal anti-delta antibody product of a somatic cell hybrid. These antibodies induce B cells to proliferate. Structural studies will be undertaken to define the region of the delta chain in which the target determinant is located. Experiments will be performed to investigate whether these anti-delta antibodies stimulate a unique subpopulation of B lymphocytes. The ability of the sIgD receptor to deliver an adquate signal for full differentiation will be investigated. Finally, the intracellular biochemical events resulting from stimulation with anti-immunoglobulins will be examined.